The present invention relates to lens aberrations, and more particularly to the lens aberrations in lithography.
Lithography, a well known process in the art, is often used to print patterns onto a silicon wafer. In lithography, a device called the xe2x80x9cstepperxe2x80x9d emits light through a lens. The light is then emitted through a patterned mask, projecting the pattern onto the wafer. The projected pattern is then printed onto the wafer. The clarity of the printed pattern depends upon the lens aberration. Because of the increasingly high density of devices on a single wafer, the clarity of the printed pattern is increasingly important. To properly control the clarity of the printed pattern, a chip manufacturer needs to have an accurate measurement of the aberration in the lens so that a stepper with an acceptable lens aberration is purchased.
Typically, the lens manufacturer, and the stepper manufacturer who installs the lens into its stepper, have calculated the lens aberration. Various conventional methods are used. Each of these methods use xe2x80x9cZernike polynomialsxe2x80x9d. The Zernike polynomials are a set of equations which represent the effects of various types of lens aberrations. The sum of all of the Zernike polynomials give the total aberration of the lens. Each of the Zernike polynomials has a coefficient. By measuring the effects of the aberrations, one or more of the Zernike polynomial coefficients may be calculated. One type of aberrations is referred to as xe2x80x9ccomaxe2x80x9d. Coma results from unequal bending of parallel light rays from an off-axis object. The effects of coma is image asymmetry and pattern shift. The amount of image asymmetry is referred to as the xe2x80x9ccritical dimensionxe2x80x9d, or xe2x80x9cCDxe2x80x9d, difference. The CD difference is measured by a CD measuring (CDM) tool to determine the amount of asymmetry between two locations of a pattern projected through the lens. Zernike polynomials are well known in the art and will not be further described here.
Conventional methods used to calculate the aberration of lenses in steppers include measuring the wavefront deviation using an interferometer and measuring the relative phase shift of a projected pattern. However, the problem with the methods is they do not measure the lens aberration based on light on the wafer, i.e., they do not measure the xe2x80x9crealxe2x80x9d lens aberration from the pattern printed on the wafer. Also, for these conventional methods, a projected reference pattern is required to represent the xe2x80x9cperfectxe2x80x9d pattern. But since the reference pattern is projected, it is affected by lens aberrations as well. Thus, only a relative lens aberration can be measured. This does not provide the chip manufacturer with enough quality control over the printed patterns on a wafer.
Accordingly, there exists a need for a method for measuring lens aberration of light on a wafer. The method should not require a projected reference pattern. The present invention addresses such a need.
The present invention provides a method for measuring lens aberration of light on a wafer. The method includes printing a pattern on the wafer by projecting the pattern through a lens in a plurality of pitches and directions; measuring a plurality of critical dimension (CD) differences between two locations on the printed pattern for each of the plurality of pitches and directions; and determining at least one Zernike coma aberration coefficient based on the measured plurality of CD differences. The method in accordance with the present invention measures the CD difference between two locations on the printed pattern on a wafer. This CD difference is then used to calculate the Zernike coma aberration coefficients. No projected reference pattern is required to measure the CD difference, and thus an absolute coma aberration can be calculated. Also, the coma aberration coefficients are based on the light projected onto the wafer, allowing chip manufacturers to more precisely select a stepper with an appropriate lens aberration. This in turn allows better quality control in the clarity of patterns printed on wafers.